Method for forming a metal cap in a semiconductor memory device

ABSTRACT

Exemplary embodiments of the present invention are directed towards a method for fabricating a semiconductor memory device comprising selectively depositing a material to form a cap above a recessed cell structure in order to prevent degradation of components inside the cell structure in oxidative or corrosive environments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/283,539, filed May 21, 2014, the entire disclosures of which ishereby incorporated herein by reference.

FIELD

Certain embodiments of the disclosure relate to forming a semiconductordevice. More specifically, embodiments of the disclosure relate to amethod for forming a metal cap in a semiconductor memory device.

BACKGROUND

Recently, the requirements on nonvolatile memory chips' storage capacityand power consumption have been advancing rapidly. Development ofminiaturized and high-speed semiconductor elements is progressing at asimilar pace. Resistance varying memory device are being used more oftento replace flash memory devices. A resistance varying memory deviceemploys a variable resistance element as a storage element. Theresistance varying memory device includes, but is not limited to, ReRAM(Resistive RAM), CBRAM (Conductive Bridging Random Access Memory), phasechange RAM (PCRAM), and the like. In a typical resistive memory device,cell material is deposited on a substrate using a conventional method,such as CVD, PVD, or plating. The cell material can be homogenous ornon-homogeneous, e.g., consisting of multiple layers. The substrate,including bottom electrodes, can be homogenous or nonhomogeneous, e.g.,a combination of metal contacts and a dielectric. A conductive metallayer is deposited on the cell material, acting as the top electrode.Generally, the cell material resistance is changed, for example, from ahigh-resistance state to a low- resistance state, when a certain voltageis applied to the top electrode.

However, it is difficult to make such a memory cell structure usingphotoresist masking and plasma etching, because the cell material tendsto be complex and may, for example, consist of multiple transitionalmetal elements. Thus, these advanced cell materials cannot be patternedeasily by commercially available dry etching methods.

Accordingly, a damascene process is generally used to fabricate the cellstructures, where a thick layer (acting as a template layer) isdeposited on a substrate and patterned with open trenches where cellmaterial will be deposited. A thick coating of the cell material is thendeposited that significantly overfills the trenches in the templatelayer. The excess cell material above the template layer (referred to asoverburden) is removed through chemical-mechanical planarization (CMP),and the template layer is exposed. The template layer is selectivelyremoved or exhumed via a chemical method, a wet strip or a dry strip,and isolated cell structures arefabricated.

However, during the exhumation, the top of the cell materials isimpacted or damaged through oxidization or corrosion because of thedry/wet chemical reactions. This degrades the top electrode conductivityand the cell material performance.

Therefore, there is a need in the art for a method for forming asemiconductor memory device without degradation of the cell material inaccordance with exemplary embodiments of the present invention.

SUMMARY

A method for forming a metal cap in a semiconductor memory device isprovided substantially as shown in and/or described in connection withat least one of the figures, as set forth more completely in the claims.

These and other features and advantages of the present disclosure may beappreciated from a review of the following detailed description of thepresent disclosure, along with the accompanying figures in which likereference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 depict a method for forming a metal cap in a semiconductormemory device in accordance with exemplary embodiments of the presentinvention.

FIG. 1 depicts a first step in a fabrication process in accordance withexemplary embodiments of the present invention;

FIG. 2 depicts a second step in the fabrication process in accordancewith exemplary embodiments of the present invention;

FIG. 3 depicts a third step in the fabrication process in accordancewith exemplary embodiments of the present invention;

FIG. 4 depicts a fourth step in the fabrication process in accordancewith exemplary embodiments of the present invention;

DETAILED DESCRIPTION

Certain implementations may be found in a selectively formed metal capin a memory cell structure. According to one embodiment, a memory cellcan be formed with a damascene process. Namely, a template layer ispatterned and etched to form trenches, active cell material and a firstmetal layer are deposited forming structures in the trenches, and thenthe structures in the trenches are isolated by a chemical-mechanicalplanarization (CMP). The exposed first metal surface is cleaned andselectively covered with a cap which comprises a second metal. Thesecond metal has some resistance to oxidation and/or corrosion. Thetemplate layer is selectively removed by plasma enhanced oxidation,known as exhumation, or by chemical dissolving. The top surface of theactive cell material and the top electrode are protected by the caplayer during template layer removal. The memory cell may be resistiveRAM (ReRAM), conductive-bridge RAM (CB-RAM) cell, phase change memory(PCRAM), or the like.

FIGS. 1-4 depict a method for forming a metal cap in a semiconductormemory device 100 in accordance with exemplary embodiments of thepresent invention.

FIG. 1 depicts a first step in a fabrication process of the device 100.The result of a damascene process is the structure shown in FIG. 1,where a substrate 102 has a first contact 112 and a second contact 114(bottom electrode contacts—BEG). Those of ordinary skill in the art willrecognize that the substrate 102 may contain a single contact, or aplurality of contacts, depending on the usage of the resulting device100 and the present invention does not limit the device 100 to havingmerely two contacts. The metal contacts 112 and 114 are electricallyconductive, acting as bottom electrodes, and are fabricated through asubstrate layer 102 which contains dielectric material such as siliconoxide, silicon nitride, or the like. This dielectric layer 102 separatesthe active cell material 108 from an active semiconductor base material(not shown in FIG. 1, but well known to those of ordinary skill in theart). The active semiconductor base material can be a terminal, forexample, a gate terminal, of a transistor. In some embodiments such atransistor is a metal-oxide-semiconductor field-effect transistor(MOSFET) for amplifying or switching electronic signals. A templatelayer 104 sits atop the substrate 102. A trench 105 is formed in thetemplate layer 104 during the damascene process. Cell material 108 isdeposited on and lines the trench 105. A first metal layer 110 is formedabove the cell material 108 to fill the trench 105. The first metallayer 110 will form the top electrode. In some embodiments, copper ischosen as the first metal layer 110 to achieve a low resistance cellmetal line. In other embodiments, the metal layer 110 may be Cu, Au, Ta,Ru, Pt, W, Ti, Poly, or the like. The template layer 104 may becomprised of Carbon, PolySilicon, Silicon oxide, or the like.

FIG. 2 depicts a second step in the fabrication process in accordancewith exemplary embodiments of the present invention. Chemical-mechanicalplanarization (CMP) is performed to remove the bulk of the first metallayer 110 and a portion of the cell material 108. The planarized firstmetal layer 110 and the cell material 108 form the cell structure 202,which is recessed from the plane of the template layer 104. According toone embodiment, the CMP process uses traditional CMP abrasives such ascolloidal silica, fumed silica, or colloidal alumina. The first metallayer 110 and the cell material 108 are removed by mechanical actionsand/or chemical modification during the CMP. When the template layer 104is exposed by CMP, the friction between the wafer surface and thepolishing pad might change significantly, and a friction-basedendpointed process control method becomes feasible. Similarly, wafersurface optical reflection strength might change significantly when thetemplate layer 104 is exposed by CMP and this makes an opticalfriction-based end-pointed process control method feasible. After thechange in process traces (such as friction trace or optical reflectiontrace) is captured, over-polishing begins. Over-polishing results in thecell material 108 receding beneath the plane of the template layer 104,leaving a recess 200. The depth of the recess 200 is mostly determinedby the over-polish time. Over polishing also fully exposes the surfaceof the template layer 104 so that it will be consumed later with anexhumation method. After the CMP, a wet clean process is performed toremove any oxidized surface metal and to expose a fresh surface of thefirst metal layer 110 in the trench 105. The fresh surface aids in theselective deposition the cap material in recess 200 at a later stagedescribed in FIG. 3. In some embodiments, the cap material to bedeposited in recess 200 is W or Ti, both of which have strongerresistance to corrosion than copper (namely, the first metal layer 110)or the active cell material 108. The cell structure 202 is more stablewhen a wet clean process is applied onto the wafer after the templatelayer 104 is exhumed, in order to remove various polymer residualsgenerated during plasma enhanced exhumation. According to someembodiments, a plasma-based dry etch method selectively removes thetemplate layer 104 while the cell material 108 remains. In some cases,highly active chemical species may be generated when the template layer104 is exhumed, producing solid byproducts over the surface of the cellstructure 202. According to one embodiment, these by-products areselectively removed by a wet clean method using some acidic or basicaqueous solution. This process is generally referred to as the post-etchclean, well known to those of ordinary skill in the art. Copper issensitive to this clean. W and Ti are immune to such wet clean.

FIG. 3 depicts a third step in the fabrication process in accordancewith exemplary embodiments of the present invention. A material isselectively deposited as a cap 300 in the recess 200 on the freshsurface of the first metal layer 110, to protect the cell structure 202,and specifically to protect the first metal layer 110 from any chemicalreaction that may be caused when the template layer 104 is selectivelyremoved. The cap 300 is only deposited in the recess 200 and is notdeposited on the template layer 104. The template layer 104 is stillexposed after cap 300 formation. In some embodiments, the cap 300 may beformed using W, Ti, Co, or the like, able to be selectively depositedover the first metal layer 110. As an example, according to oneembodiment, the selective depositing of the elemental tungsten is bychemical vapor deposition within a deposition chamber using gaseous WF6and SiH4 as deposition precursors which are fed to the chamber duringthe deposition. Inert and/or other gases may be fed to the chamberduring the deposition. In this embodiment during the deposition,substrate temperature is from approximately 250° C. to 350° C., chamberpressure is from approximately 1 mTorr to 100 mTorr, WF6 flow rate tothe chamber is from approximately 10 seem to 1,000 seem, and SiH4 flowrate to the chamber is from approximately 5 seem to 50 seem. An exampleinert gas flow rate (e.g., Ar) is approximately 0 seem to 1,000 seem.Those of ordinary skill in the art will recognize that these ranges arenot specifically required and other ranges may be used in theimplementation of the present invention.

A W Cap or Ti Cap has resistance to corrosion during a wet clean, whichfurther prevents corrosion of the first metal layer 110 (e.g., a Culayer). In some embodiments, the cap 300 is a selective dielectric capusing materials such as SiN, SiOx, and high-k dielectrics such as HfOx,AIOx, ZrOx, and the like. According to exemplary embodiments, theselective metal or selective dielectric is deposited by one of chemicalvapor deposition (CVD), atomic layer deposition (ALO), selectiveelectroless plating or the like. In some embodiments, the cap 300 can besacrificial, i.e., the cap 300 can be selectively removed by dry etchtechnology after template layer 104 exhumation.

FIG. 4 depicts a fourth step in the fabrication process in accordancewith exemplary embodiments of the present invention. The template layer104 is selectively removed via a dry process or a wet process. Accordingto exemplary embodiments, if the template layer 104 is carbon,oxygen-containing plasma is used for the exhume process. If the templatelayer is polysilicon, exhumation can be performed using F-containingplasma dry etch. In another embodiment where the template layer 104 iscomprised of silicon oxide, an HF containing aqueous solution is usedfor the exhumation process. Those of ordinary skill in the art willrecognize that the exhumation process will conform to the material usedto form the template layer 104. Due to the formation of the cap 300above the first metal layer 110, the top surface of the metal layer 110in the cell structure 202 is prevented from oxidation or corrosionduring exhumation and is well preserved as compared to exhume processeswhere the copper layer is not protected. However, part of the side wallof the active cell material 108 might be oxidized or chemicallymodified, shown as the sidewall film 400 in FIG. 4. The sidewall film400 becomes a barrier to isolate the inner cell structures 108 and 110from various chemical reactions outside. While the present disclosurehas been described with reference to certain embodiments, it will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted without departing from the scope ofthe present disclosure. In addition, many modifications may be made toadapt a particular situation or material to the teachings of the presentdisclosure without departing from its scope. Therefore, it is intendedthat the present disclosure not be limited to the particular embodimentdisclosed, but that the present disclosure will include all embodimentsfalling within the scope of the appended claims.

What is claimed is:
 1. A device comprising: a recessed cell structure formed in a trench atop a substrate, the trench created by a damascene process; and a cap formed above the recessed cell structure to protect the recessed cell structure from degradation.
 2. The device of claim 1 wherein the cap is formed by a selective deposition.
 3. The device of claim 2 forming a memory cell, wherein the memory cell is a resistive RAM (ReRAM) cell, conductive-bridge RAM (CB-RAM) cell, or a phase change memory (PCRAM) cell.
 4. The device of claim 2 wherein the cap is a dielectric cap or a metal cap.
 5. The device of claim 4, wherein the metal cap is formed using one of W, TiN, TaN, Ta or TiW, and the dielectric cap is formed using one of SiN, SiOx, or a high-k dielectric. 